In resent years, high density mounting of semiconductor devices is in need for, due to the multiple functioning, downsizing, weight reduction and reduction in the thickness of portable information devices. Consequently, an MCM structure is proposed in order to meet these demands. The MCM realizes one package in which a plurality of semiconductor devices are mounted.
An example of a semiconductor device having the MCM structure is disclosed in Japanese Unexamined Patent Publication, Tokukaihei, No. 6-89962 (published on Mar. 29, 1994). FIG. 15 is a plan view illustrating the structure of a conventional semiconductor device 100, and shows the state which has not been subjected to sealing with a resin. FIG. 16 is a side sectional view of the conventional semiconductor device 100, which has been subjected to the sealing with the resin, taken along line E-E in FIG. 15.
The semiconductor device 100 includes two semiconductor chips 101, which are juxtaposed. The two semiconductor chips 101 are directly mounted on tabs 102, respectively, and the tabs 102 are linked together by a tab joining section 103. Each of the semiconductor chips 101 receives and outputs a signal, by being wire bonded with a lead 104 via a bonding wire 105. In addition, a sealing resin 106 seals the two semiconductor chips 101 and the bonding wire 105.
By thus packing the semiconductor device including an MCM structure (MCM packaging), it is not necessary for one semiconductor device (SoC (System on Chip)) to be integrated so as to have a plurality of functions. As such, it is advantageous to cost and yield rate of fabrication of the semiconductor device.
In addition, when comparing a case where a plurality of semiconductor chips, each having a respective function, are separately packaged with a case where a plurality of semiconductor chips, each having a respective function, are MCM-packaged, the latter case can have less mounting area than the former, when mounting the semiconductor chips.
However, the conventional semiconductor device 100 has the following problem. A warpage is readily generated, if the semiconductor device 100 is of a slim package with a high aspect ratio. The aspect ratio is a ratio indicated by X/Y, where X and Y are a long side and a short side, respectively, of a two-dimensional shape.
FIG. 17 is a view illustrating a warpage which is generated in an MCM-packaged semiconductor device 110. As illustrated in FIG. 17, a warpage W1 is generated in a direction perpendicular to the mounting surface on each end part of the semiconductor device 110, when the mounting surface of the semiconductor device 110 has a size of X×Y. In addition, a bending point P emerges due to this warpage.
FIG. 18 is a view illustrating a warpage generated in a semiconductor device 120, which is of a larger size than that of the semiconductor device 110 in FIG. 17. As illustrated in FIG. 18, a warpage W2 is generated in a direction perpendicular to the mounting surface on each end part of the semiconductor device 120, when the mounting size of the semiconductor device 120 has a size of 2X×Y. In addition, a bending point P emerges due to this warpage, as similar to the semiconductor device 110.
If the semiconductor device 110 in FIG. 17 and the semiconductor device 120 in FIG. 18 warp with the same curvature, the warpage W2 of the semiconductor device 120 in FIG. 18 with a higher aspect ratio will be greater than the warpage W1 of the semiconductor device 110 in FIG. 17 (W1<W2). That is to say, the warpage greatens with a higher aspect ratio, even if the curvature is the same.
However, there is a high demand for extremely thin packages, particularly from the reduction in the thickness of portable devices itself. Thus, solving the problem of warping is demanded even more.
The following two points appear to be the cause for the warpage which is generated in the MCM-packaged semiconductor device.
The first cause is that the MCM-packaged semiconductor device tends to warp concavely as a whole from the cure shrinkage of a sealing resin. Particularly, in case of a slim semiconductor device, the cure shrinkage force of the resin is greater than the rigidity of the semiconductor device. Thus, a large concave warpage occurs. In contrast, if a resin with a lower cure shrinkage force is used, a convex warpage is generated as in an MCM-packaged semiconductor device 130, as illustrated in FIG. 19.
The second cause is that the bending point of the warpage tends to be generated particularly in an area between the semiconductor chips which are juxtaposed, as illustrated in FIG. 20. FIG. 20 is a view illustrating bending points P which emerge when a warpage is generated, in a semiconductor device 140 in which three semiconductor chips 141, 142 and 143 are juxtaposed. The bending points P emerge between the semiconductor chips 141 and 142, and between the semiconductor chip 141 and 143, respectively. The warpage W3 generates at each end part of the semiconductor device 140. The warpage starts to bend from the bending points P.
The trend of the second cause appears to be based on the fact that no semiconductor chip exists. It appears that the warpage is generated from the following reasons: (i) since the area with no semiconductor chip has a great amount of resin, a great amount of cure shrinkage occurs; and (ii) the rigidity is also insufficient since there is no semiconductor chip. In addition, a warpage (W4) at the end part of a semiconductor device 150 becomes greater as the bending points P are closer to the center of the semiconductor device 150, as illustrated in FIG. 21. Specifically, the warpage for the semiconductor device 150 satisfies W3<W4.
The generation of a large warpage is likely to cause inadequate mounting. That is, a problem of inadequate connection occurs when mounting the semiconductor device to a substrate or the like, if the warpage becomes greater.
For example, in FIG. 18, the semiconductor device 120 is mounted on a mounting substrate 125, while positioning of solder balls 121 supplied to the external terminal of the semiconductor device 120 with respect to a connection pad 126 supplied with a solder paste 127 on the mounting substrate 125. If the warpage is great in the semiconductor device 120, the solder paste 127 and the solder balls 121 cannot come into contact with each other. There is a high possibility that inadequate connection would occur if the solder is melted by a reflow furnace in this state.